1. Field of the Invention
The present invention relates in general to the control of a video bit rate, and more particularly to a video bit rate control method for controlling the video bit rate effectively even upon occurrence of a sudden scene change.
2. Description of the Prior Art
Generally, in the case of coding motion pictures, the generated bits for each frame must be varied according to a frame complexity to maintain a picture quality constant. However, in the case of coding the motion pictures to transmit them, a coded output bit rate is required to be maintained constant because coded data must be transmitted through constant channel bandwidth.
In order to maintain the coded output bit rate constant, bits necessary to the coding must be controlled properly every frame. In this case, a degradation in the picture quality may become an issue. As a result, there has been proposed an effective video bit rate control method which is capable of avoiding an abrupt degradation in the picture quality and maintaining the coded output bit rate constant within the transmission channel band.
Referring to FIG. 1, there is shown a block diagram of a general video bit rate control apparatus. As shown in this drawing, the video bit rate control apparatus comprises a video frame input unit 1 for inputting a video frame, a coder 2 for coding the video frame by the video frame input unit 1, and a buffer 3 for storing temporarily the coded data from the coder 2 and transmitting the stored data externally according to a constant transmission bit rate. The video bit rate control apparatus further comprises a video bit rate controller 4 for controlling an output bit rate of the coder 2 according to a storage level of the buffer 3 to prevent an overflow or an underflow of the buffer 3.
Referring to FIG. 2, there is shown a flowchart illustrating a conventional video bit rate control method. constant bits are first assigned according to a limited transmission rate to each of a certain number of frames of a group of pictures (referred to hereinafter as GOP) to be coded. The subsequent bit assignment is performed in the frame coding order in consideration of characteristics of the frames and the remaining bits of every frame.
Namely, at the step 201, assignment bits are calculated with respect to the frames in the GOP to be coded, in consideration of a bit redundancy of the GOP, the frame characteristics and the number of the frames remaining in the GOP, not coded. At the step 202, assignment bits are calculated in the unit of block according to assignment bits for frames and the storage level of the buffer 3 at the time point that blocks are coded. As a result of the calculation of the assignment bits at the step 202, quantization parameters are obtained at the step 203. Then at the step 203, the resultant assignment bits are transferred to the coder 2.
It is checked at the step 204 whether a current block is a last one in the current frame to be coded. If it is checked at the step 204 that the current block is not the last one in the current frame to be coded, the assignment bits are calculated in the unit of block up to the last block in the current frame at the step 202 and the quantization parameters are obtained at the step 203. Namely, the steps 202 and 203 are repeatedly performed up to the last block in the current frame. On the contrary, if it is checked at the step 204 that the current block is the last one in the current frame, the above operation is repeatedly performed with respect to a new frame.
By the way, in the above-mentioned conventional video bit rate control method, in the case where a sudden scene change occurs in the middle of coding the frames in the GOP, much bits are required to maintain the picture quality constant. In this case, the remaining bit amount is used to code a scene change frame and the subsequent frames because the constant bit amount was used for the frames previously coded. As a result, there is caused a degradation in the picture quality in any form. This degradation in the picture quality has a bad effect on a high definition television application.